1. Field of the Invention
The present invention relates to a display device using an organic light emitting device that has an anode, a cathode, and a film containing an organic compound that emits light by application of electric field (hereinafter referred to as organic compound film). Specifically, the present invention relates to a full color display device using a blue organic light emitting device or a white organic light emitting device that has higher light emission efficiency and longer lifetime than conventional ones. The term display device in this specification refers to an image display device that employs as a light emitting device an organic light emitting device. Also included in the definition of the display device are: a module in which a connector, such as an anisotropic conductive film (FPC: flexible printed circuit), a TAB (tape automated bonding) tape, or a TCP (tape carrier package), is attached to an organic light emitting device; a module in which a printed wiring board is provided on the tip of a TAB tape or a TCP; and a module in which an IC (integrated circuit) is mounted directly to an organic light emitting device by the COG (chip on glass) method.
2. Description of the Related Art
An organic light emitting device is a device that emits light when electric field is applied. Light emission mechanism thereof is said to be as follows. A voltage is applied to an organic compound film sandwiched between electrodes to cause recombination of electrons injected from the cathode and holes injected from the anode in the organic compound film and, when the resultingly excited molecule (hereinafter referred to as molecular exciton) returns to base state to release energy in the form of light emission.
There are two types of molecular excitons from organic compounds; one is singlet exciton and the other is triplet exciton. This specification includes both cases where singlet excitation causes light emission and where triplet excitation causes light emission.
In an organic light emitting device such as the above, its organic compound film is usually a thin film with a thickness of less than 1 μm. In addition, the organic light emitting device does not need back light used in conventional liquid crystal displays because it is a self-light emitting device and the organic compound film itself emits light. The organic light emitting device is therefore useful in manufacturing a very thin and light-weight display device, which is a great advantage.
When the organic compound film is about 100 to 200 nm in thickness, for example, recombination takes place within several tens nanoseconds after injecting carriers, based on the mobility of the carriers in the organic compound film. Considering, the process from carrier recombination to light emission, the organic light emitting device is readied for light emission in microseconds. Accordingly, quick response is also one of the advantages of the organic light emitting device.
Since the organic light emitting device is of carrier injection type, it can be driven with a direct-current voltage and noise is hardly generated. Regarding a drive voltage, a report says that a sufficient luminance of 100 cd/m2 is obtained at 5.5 V by using a very thin film with a uniform thickness of about 100 nm for the organic compound film, choosing an electrode material capable of lowering a carrier injection barrier against the organic compound film, and further introducing the hetero structure (two-layer structure) (Reference 1: C. W. Tang and S. A. VanSlyke, “Organic electroluminescent diodes”, Applied Physics Letters, vol. 51, no. 12, 913-915 (1987)).
With those features, including being thinner and lighter, quick response, and direct current low voltage driving, an organic light emitting device is attracting attention as a next-generation flat panel display device. In addition, with being a self-light emitting type and a wide viewing angle, the organic light emitting device has better visibility and is considered as effective especially in using for a display screen of portable equipment.
Another feature of organic light emitting devices is emission of light of various colors. The well varied colors are derived from the diversity of organic compounds. In other words, the various colors are derived from the flexibility, with which materials emitting different colors can be developed by designing a molecule (introduction of a substituent, for example).
From these points, it is safe to say that the most promising application field of organic light emitting devices is in full color flat panel displays. Various methods have been devised to display full color while considering the characteristics of organic light emitting devices. Currently, there are three major methods for manufacturing a full color display device using an organic light emitting device.
One of those major methods is to separately form an organic light emitting device that emits red light, an organic light emitting device that emits green light, and an organic light emitting device that emits blue light using a shadow mask technique. Red, green, and blue are the primary three colors of light, and each of the three types of organic light emitting devices makes one pixel. This method is hereinafter referred to as a separate formation method. Another one of the major methods obtains the primary three colors of light by using a blue organic light emitting device as a light emission source and converting the blue light into green light and red light through color conversion layers (CCM) that are formed of organic fluorescent materials. This method is hereinafter referred to as a CCM method. The last one is a method of obtaining the primary three colors of light by transmitting white light from a white organic light emitting device used as a light emission source through color filters (CF) that are used in liquid crystal display devices or the like. This method is hereinafter referred to as a CF method.
The separate formation method is most efficient in taking out emitted light since the method does not suffer light loss in light conversion layers of the CCM method (the conversion efficiency hardly reaches 100%), or light absorption by color filters in the CF method. The separate formation method is an appealing method in this aspect, for the method allows a display device to fully benefit from the characteristics of self-luminous organic light emitting devices.
However, the separate formation methods also have some problems. For example, the shadow mask used in this method finds difficulties in dealing with a pixel that is smaller in size. Furthermore, the mask has to change locations every time the manufacture proceeds from formation of an organic light emitting device for one color to formation of an organic light emitting device for another color. The operation of changing locations of the mask is rather onerous and leads to unsatisfactory productivity.
A more serious problem of the separate formation method is that, at present, characteristics (light emission efficiency and lifetime) vary between a red light emitting device, a green light emitting device, and a blue light emitting device.
As to the light emission efficiency, for example, the lowest required efficiency (equals to power efficiency, the unit thereof is 1 m/W) is proposed for each of the primary three colors of light in full color display (Reference 2: Yoshiharu Sato, “Journal of Organic Molecules and Bioelectronics Division of The Japan Society of Applied Physics”, vol. 11, no. 1, 86-99 (2000)). According to Reference 2, there are many reports in which a green light emitting device and a blue light emitting device exhibit light emission efficiency exceeding their respective required values. On the other hand, the light emission efficiency of red light emitting device falls far below its required value. Accordingly, under the present circumstances, low light emission efficiency of red light emitting device is the stumbling block to a full color display device by the separate formation method.
As to the lifetime (lowering of luminance with time), it is rare that the lifetime of an organic light emitting device of one color exactly coincides with the lifetime of an organic light emitting device of another color. This means that the color balance among the primary three colors of light could be lost with time resulting in inaccurate coloring and irregular luminance, which are fatal defects as a display.
On the other hand, a merit of the CCM method and the CF method is that the methods do not have the fatal problems of the separate formation method as described above despite their rather inferior efficiency in taking out emitted light due to slight loss or absorption of light.
The CCM method or the CF method does not need the minute operation for separately forming organic light emitting devices of different colors using a shadow mask since an organic light emitting device of a single color (blue in the case of the CCM method, white in the case of the CF method) are used. Also, a color conversion layer and a color filter can be formed by a conventional photolithography technique and no complicate process is necessary. Moreover, the CCM method and the CF method are free from inaccurate coloring and irregular luminance over time because only one type of organic light emitting device is used to cause the luminance to change uniformly with time.
From the above, the CCM method and the CF method can be very effective methods in manufacturing a full color display device if it is possible to obtain a blue or white organic light emitting device that has higher luminance and longer lifetime.
However, blue organic light emitting devices and white organic light emitting devices have several problems. First, shortness of lifetime can be given as a problem common to the two.
Blue organic light emitting devices have made an exponential advance in recent years as a result of development of a distyryl arylene-based blue light emitting material. The material makes it possible for the luminance to achieve a half-life of 20 thousand hours when the initial luminance is set to 100 cd/m2 and the device is driven with a constant current (Reference 3: Masatoshi Aketagawa, “Monthly Display, October 1998, Special Issue on Organic EL Display, 100-104”).
Despite this advancement, a blue light organic light emitting device needs to emit blue light with even higher luminance in order to obtain bright green light and red light in realizing a full color display using the CCM method (because of loss by a color conversion layer). The lifetime of an organic light emitting device becomes shorter as the device emits light at higher luminance. Therefore, when the CCM method is used, the lifetime has to be even longer. For example, green organic light emitting devices that have the longest lifetime can last fifty thousand hours if the conditions are the same, and blue organic light emitting devices are desired to achieve lifetime of this long.
The problem of short lifetime is more serious for white organic light emitting devices. A report says that, except one sample, the half-life of the luminance of white organic light emitting devices formed from low molecular weight materials is on the order of several tens hours when the initial luminance is set to 100 cd/m2 and the devices are driven with a constant current (Reference 4: Yasuhisa Kishikami, “Monthly Display”, September 2000, 20-25).
Low light emission efficiency is another problem of white organic light emitting devices. In the CF method where white organic light emitting devices are combined with color filters, the light emission efficiency is fatally low since a large portion of emitted light is absorbed by the color filters. High light emission efficiency is desired also in the CCM method where blue organic light emitting devices are used since loss of light is caused due to color conversion layers.